Owing to the recent progress in micro-processing technology, liquid crystal material technology and mounting technology, liquid crystal panels of 5 to 50 cm in diagonal size are commercially presented as practicable display devices of television image and various images. Color display is also realized easily by forming coloring layers of R, G, B in one of the two glass substrates for composing a liquid crystal panel. In particular, in the so-called active type liquid crystal panel having switching elements incorporated in each pixel, images of small crosstalk, fast response, and high contrast ratio are guaranteed.
In such liquid crystal panels, the matrix is generally composed of 100 to 1,000 scanning lines and 200 to 2,000 signal lines, but lately trends for larger screen size and higher definition are promoted simultaneously.
FIG. 8 shows a mounting state on a liquid crystal panel in a prior art. This liquid crystal panel 1 is realized by various mounting methods, such as COG (chip on glass) method in which a semiconductor integrated circuit chip 3 for supplying drive signals is directly connected to an electrode terminal group 6 of scanning lines formed on one transparent insulating substrate (for example, glass substrate) 2, or TCP (tape carrier package) method in which a TCP film 4 having terminals (not shown) of gold-plated copper foils are fixed to a terminal group 5 of signal lines by pressing with an adequate adhesive containing a conductive medium, using, for example, a polyimide resin thin film as the base, and therefore electric signals are supplied into the image display unit. In the drawing, two mounting methods are shown simultaneously, but actually either method is selected appropriately.
Reference numerals 7 and 8 are wiring routes for connecting between the image display unit of the liquid crystal panel 1 and electrode terminal groups 5 and 6 of signal lines and scanning lines, and they may not be always composed of same conductive materials as the electrode terminal groups 5 and 6.
Reference numeral 9 is other transparent insulating substrate (for example, glass substrate) having a counter electrode of transparent conductivity common to all liquid crystal cells. Two substrates 200 and 900 for composing the liquid crystal panel 1 are disposed at a specified gap of about several microns through a spacer member such as resinous fiber or bead. This gap is a closed space as being closed by a seal member and a stopping member made of organic resin in the peripheral parts of the substrates 200, 900, and this closed space is filled with liquid crystal.
To realize a color display, generally, the closed space side of the glass substrate 9 is coated with an organic thin film in a thickness of about 1 to 2 .mu.m containing either one or both of dye and pigment called coloring layer, and a color display function is provided, and in such a case the color substrate 900 is particularly called "color filter substrate." Depending on the properties of liquid crystal material, a polarizer is adhered to either one or both of the upper surface of the glass substrate 9 and the lower surface of the glass substrate 2, and the liquid crystal panel 1 functions as an electro-optic element. A general twisted nematic (TN) liquid crystal requires two polarizers.
FIG. 9 is an equivalent circuit diagram of an active type liquid crystal panel having, for example, thin film insulating gate type transistors disposed at each pixel as conventional switching elements. Elements indicated by solid line are switching elements formed on one glass substrate 2, and compose the active substrate 200, and elements indicated by broken line are formed on other glass substrate 9, and compose the color filter substrate 900. Scanning lines 11 (8) and signal lines 12 (7) are formed on the active substrate 200 simultaneously when forming a TFT (thin film transistor) 10 using, for example, amorphous silicon as semiconductor layer and silicon nitride layer as gate insulating layer.
Liquid crystal cells 13 are composed of transparent conductive pixel electrodes 14 (see FIG. 10) formed on the active substrate 200, transparent conductive counter electrodes 15 formed on the color filter substrate 900, and liquid crystal 16 (see FIG. 10) contained in the closed space formed between the two substrates 200 and 900, and electrically they are equivalent to capacitors. The constitution of storage capacity for increasing the time constant of the liquid crystal cells 13 may be realized by several methods. In FIG. 9, for example, a storage capacity 17 is composed by inserting an insulating layer such as gate insulating layer of TFT 10 between common electrodes 18 common to all pixel electrodes 14 and the pixel electrodes 14.
FIG. 10 is a sectional view showing essential parts of an active type color liquid crystal panel. Coloring layers 19 composed of stained photosensitive gelatin or coloring photosensitive resin are disposed in a specified arrangement in three primaries of R, G, B, corresponding to the pixel electrodes 14, at the closed space side of the color filter substrate 900. Counter electrodes 15 common to all pixel electrodes 14 are formed on the coloring layers 19 in order to avoid loss of voltage distribution in the liquid crystal cells due to interposition of the coloring layers 19. Orientation films (for example, polyimide resin thin film layers in a film thickness of about 0.1 .mu.m) 20 applied on the two substrates 200, 900, contacting with the liquid crystal 16 are intended to align the liquid crystal molecules in a specified direction. Thus, when the twisted nematic (TN) type is used as the liquid crystal 16, upper and lower polarizers 21 are needed.
When opaque films 22 of low reflectivity are arranged in the boundaries of R, G, B coloring layers 19, reflected light from the wiring layers such as signal lines 12 on the active substrate 200 can be prevented, and therefore the contrast ratio of the image is enhanced. It is also possible to prevent increase of leak current during OFF operation due to external light irradiation of the TFT 10 which is a switching element, 80 that the liquid crystal panel can be operated even under an intense external light. This is already realized as black matrix (BM).
Various examples may be also considered for the composition of the black matrix (BM). From the viewpoint of cost, it is not advantageous in consideration of occurrence of step in the boundaries of adjacent coloring layers 19 and transmissivity of light, therefore, it is easy and rational to use metal thin films in a film thickness of about 0.1 .mu.m, for example, Cr thin films, as the opaque films 22.
In FIG. 10, meanwhile, in addition to the TFT 10, scanning lines 11 and storage capacity 17, other constituent elements such as back light source and spacer are omitted. Reference numeral 23 is a conductive thin film for connecting between the pixel electrodes 14 and the drain of the TFT 10. This conductive thin film 23 is generally formed simultaneously by the same material as the signal lines 12, and is called a drain wiring (electrode). Although not shown in the drawing, the counter electrode 15 is connected to a proper conductive pattern on the active substrate 200 having the TFT 10, through an appropriate conductive paste on the outer circumference slightly outside of the image display unit, and is incorporated in part of the electrode terminal groups 5, 6 (see FIG. 8) so as to be connected electrically.
The liquid crystal cells in this liquid crystal panel are composed of transparent conductive pixel electrodes formed on one substrate 200, similar counter electrodes on the color filter substrate 900, and liquid crystal contained in the clearance between the two substrates. In a recently developed IPS (in-plane switching) type liquid crystal panel capable of expanding the viewing field angle, liquid crystal cells are composed of a pair of comb electrodes formed on one glass substrate and liquid crystal contained in the clearance between two substrates, and therefore transparent electrodes are not required also on the color filter substrate 900, but the detail is omitted herein.
In the existing active type liquid crystal panel as mentioned herein, several to millions of unit pixels composed of switching elements and pixel electrodes are arranged, and it is obtained by the precision assembling technology of a transparent insulating substrate or active substrate such as glass substrate having electrode lines such as scanning lines and signal lines for connecting between unit pixels and electrode terminal groups formed at ends of the electrode lines and corresponding to the mounting, and other glass substrate or glass filter substrate.
Therefore, it requires nearly same materials, production facilities, environments and techniques as needed in fabrication of semiconductor integrated circuits, and actually it is realized by the manufacturing technologies applicable to micron-order pattern forming, formation of various thin films, liquid crystal cell thickness, etc.
The active type liquid crystal panel is also a device that is hard to improve the yield. Since images are displayed, evidently, not a single line defect is permitted, but it is extremely difficult to manufacture an intact product completely free from point defect, stain or speck. At the present technical level of manufacture, it is realistic to comprise at a certain extent, for example, as for point defects, a particular standard such as light point defects of 3 or less in the central area of the screen and total number of black point defects of 20 or less is specified in consideration of the panel size, resolution, etc.
Raster (white point) defects are always lit defects, and they are very obvious and hence the standard is particularly strict. Accordingly, when an active substrate is completely manufactured, electric characteristics of unit pixels are inspected, and if light point defects are discovered, a rescue technology for correcting them at this stage by the laser to convert into black point defects has been already established.
FIG. 11 shows a rescue example executed if the TFT 10 fails to operate due to some cause. This is a method of fusing the protrusion 12' of the signal line 12 insulated through a proper insulating layer and the pixel electrode 14 by using the laser and connecting them electrically as 24. Accordingly, the pixel electrode 14 is always at the same potential as the signal line 12 regardless of the control of the scanning line 12, and therefore if the display mode of the liquid crystal panel is normally white, it is known that it is not at least light point defect.
However, light point defects can occur not only when manufacturing the active substrate 200, but also when assembling the panel, and there is a limit to enhancement of yield by the correction at the time of manufacture of the active substrate 200 only. For example, pin holes of the orientation film, defective orientation and the like cannot be converted to black point defects by electric treatment. Besides, in the correction by emitting laser to the assembled liquid crystal panel, bubbles may be formed, or holes may be opened in the black matrix, and the successful rate is not always high at the present.